Why does Herbert call Pip by another name?

The two first meet when they are little boys at Miss Havisham’s house. Herbert challenges Pip to a gentlemanly fight which Pip easily wins. Pip eventually moves to London. In Jagger, the solicitor’s office, Pip is introduced to his tutor, Matthew Pocket, and his son, Herbert Pocket, a poor gentleman who wants to become a merchant. Herbert quickly nicknames Pip "Handel" because he remembers Georg Handel's work The Harmonious Blacksmith, a reference to Pip's former profession.